A method for manufacturing a device where a set of thermoelements, comprised of two junction points between two different materials, are connected in series and function as measuring sensors. The measurement is based on the physical effect that the thermoelectric voltage of a thermoelement changes as a function of which materials, such as gas or liquid, the junction points come into thermal contact with. One junction point in the thermoelements is heated by an electric heat conductor with heating current being controlled when doing so. Therefore, this junction point is referred to as xe2x80x9chot junction pointxe2x80x9d. The other junction point is not heated and is therefore referred to as xe2x80x9ccold junction pointxe2x80x9d.
In a known method (DE 40 30 401 B1) metals or semiconductors are vapor-deposited first onto foils and out of these stamped parts are cut which, by means of adhesives at the backside, are placed onto a sheet-shaped support. Subsequently, by screen printing narrow metallic conductive cement strips are applied so that thermoelements result. For a space-saving configuration L-shaped stamped parts are cut out of the foil strips and these are glued in mirror-symmetrical arrangement and with height staggering in two rows onto a support and then the ends of L-shaped legs are printed with a metallic conductive cement with lines such that serially connected thermoelements result. The manufacture of this known device is expensive and has a high failure liability. The manufacture of the stamped parts and their arrangement on the common support are cumbersome and errors are hardly avoidable. During stamping a relatively high amount of waste results which prevents an economic manufacture of the device. Alloys, such as nickel-chromium or constantan, important for the thermoelectric voltage, cannot be deposited onto the foil strip because they decompose during this process.
In another method (DE 4 34 646 A1) a stamped part in the form of a strip has also been cut out of a plastic foil which is then coated with a semi-conductor material by means of chemical, physical, or mechanical methods. On this strip a metallic conductive application layer was applied by screen printing at certain locations. This was supposed to result in large surface area connections of the thus resulting thermoelements. The thermoelements are series-connected. Onto the backside of the support a heating conductor in meander shape was printed by means of a metallic conductive cement. Because the heating current changes by a change of the surrounding temperature in the same direction, a reference sensor has been positioned on the heating conductor which controls the heating current by means of a compensation circuit. In this case, it was also not possible to use interesting alloys such as nickel-chromium or constantan neither during manufacture of the strips nor for printing of the strips.
It is an object of the invention to develop a method for a reliable manufacture of a space-saving device with high measuring precision for measuring the filling level in a container or for measuring gas by means of thermoelements. This is achieved according to the invention by the features to be explained in the following.
The invention has firstly recognized that the very interesting alloys for configuring thermoelements, chrome-nickel, on the one hand, and constantan, on the other hand, can be applied by sputtering onto a support. When sputtering, a high-energy plasma is directed onto a target where metals are impacted and are applied onto the desired carrier by means of a magnetic field. The invention uses in this connection masks where the shape of the resulting fields for the alloy for forming the thermoelements are very precisely formed by cutouts. The different masks are placed successively in a defined position so that during sputtering the different L-shaped and I-shaped fields in both rows have an extremely precise position relative to one another. On the backside of the support successively a fourth and fifth mask, either provided with a narrow or a wide slot, are provided for applying by sputtering of the desired materials, in particular, silver, a common heating conductor and its return line. The heating conductor can then be positioned precisely in the longitudinal center of the junction points between the two double rows provided on the front side of the carrier and in this way generates the common hot junction point.
In a similar way, at least one reference sensor is manufactured on a sheet-shaped support, respectively, an entire set of reference sensors. One can correct the undesirable temperature effect on the heating conductor also in other ways.
The invention also relates to a device produced according to the method according to the invention wherein the set of measuring sensors has correlated therewith a group of reference sensors.
In the known device (DE 44 34 646 A1) it was considered necessary to position the reference sensors independent of the measuring sensors in order for them not to come into contact with the liquid. If this was not done, their control effect would not have been possible. Therefore, in the known devices the reference sensor(s) have always been arranged external to the container whose filling level was to be measured. This not only requires a corresponding space expenditure but also entails circuit problems when connecting the measuring sensors and their heating conductor. The independent arrangement of the reference sensor(s) requires corresponding measures for their protection. This requires additional components which increase the space requirement. The separate attachment of the reference sensors increases also their failure liability. The manufacture and assembly of the known reference sensors and their protection are cumbersome and cost-intensive.
This is however also solved by the invention by the following measures according to which the reference sensors together with the measuring sensors are arranged in the container interior. Their interconnection is particularly simple. The container itself provides the protective function for the reference sensors arranged in its interior. The control effect of the reference sensors remains thus in effect because its junction points are brought into contact with a body having a constant thermal conductivity. This can be achieved in the form of a thermal insulation which protects the junction points of the reference sensors relative to the liquid. Such a body of constant thermal conductivity could also be a plastic body. The measuring sensors and reference sensors are insulated relative to the corresponding electric heat conductors.
Another possibility resides in that the reference sensors are arranged at the bottom of the container where there is always residual liquid present which ensures the constant thermal conductivity between the hot and cold junction points. Finally, it would also be conceivable to use for this purpose the upper area of the container where even for a full filling level no liquid will reach. In the latter case, the gas which is always present thereat provides the constant thermal conductivity. In these two last mentioned alternatives, corresponding bulges in the interior of the container are available for receiving the reference sensors in the container interior.
The simplest possibility for realizing the invention is however the aforementioned use of thermal insulation in the area of the reference sensors for which purpose different possibilities are provided. Some of them are mentioned in the dependent claims.